Background and Objectives: Favorable short- and mid-term results for hydroxyapatite (HA)-tricalcium phosphate (TCP)-coated total hip arthroplasty (THA) (Trilogy/Zimmer) have been reported in the literature; however, the long-term results beyond 15 years have not been documented. Therefore, this study evaluated the long-term postoperative results, radiological bone changes, and implant fixation of the acetabular component of HA-TCP-coated THA. Materials and Methods: This is a retrospective cohort study of 212 patients who underwent primary HA-TCP-coated THA (Trilogy/Zimmer) at our institution between 1 October 2002, and 31 March 2008; 166 who were available for follow-up at least 15 years postoperatively were included (capture rate: 78.3%). All implants were Trilogy/Zimmer. We investigated the survival rate, with aseptic loosening as the endpoint. Clinical evaluations included the presence of dislocation and a modified Harris Hip Score (mHHS) preoperatively and at the final observation. Results: The mean age at surgery and at the follow-up period were 57.7 ± 9.6 and 17.1 ± 1.5 years, respectively. The survival rate was 99.4% (165/166), with aseptic loosening as the endpoint. Dislocation was observed in 4/166 (2.4%) patients. The mHHS improved significantly from 46.1 points preoperatively to 82.2 points during the last survey (p < 0.05). The results revealed that fixation was favorable in all cases except for one case of aseptic loosening. The Trilogy implant coated with HA-TCP was highly effective in bone induction, and bone ingrowth was considered to have occurred without failure, further indicating its usefulness. The long-term results of cementless THA using an HA-TCP coating (Trilogy/Zimmer), with a mean follow-up period of 17.1 years, revealed a commendable survival rate of 99.4%, considering aseptic loosening as the endpoint. Conclusions: HA-TCP-coated THA (Trilogy/Zimmer) had good long-term results. However, further long-term observation is required in patients who have undergone this surgery, and the stem side should be evaluated and investigated, including comorbidities.